Contents & Foreword, Characterizing And ... - IRRI books

Dry matter (kg ha –1 )16,000Irrigated, measuredEarly rainfed, measuredLate rainfed, measured12,000Potential, simulatedEarly rainfed, simulatedLate rainfed, simulated8,0004,0000F M A M J J AMonthFig. 2. Simulated and measured aboveground dry matter (mean ± SE)during the 1996 walik jerami season in Jakenan. The solid line is thesimulated potential production, which may be compared with the measurementsof the best irrigated treatment. The broken and dottedlines are the simulated rainfed productions using early and late sowing.The measurements of the rainfed treatments include two tillagetreatments.ring during the reproductive phase did ORYZA overestimate grain yield by a factor of4 to 10 (measured values were 200–500 kg ha –1 while simulated values were 2,000 kgha –1 ). Whereas ORYZA partitions all assimilates produced in the reproductive phaseto the storage organs, it was observed that, in reality, when the stress was relieved, thecrop produced new tillers, indicating that not all assimilates went to the grains asmodeled.Though a statistical analysis of model performance still needs to be made, thepreliminary results indicate that ORYZA performed sufficiently well for the purposesof our study.Water table fluctuationFigure 3 presents the shallow, medium, and deep water table scenarios constructedfrom measurements in six cropping seasons of 1995-99. Large standard errors (inperiods with more than one measured value) indicated that the water table depth variedwidely in different years. The large standard errors, however, may be attributed tothe small number of available data. The mean water table depth fluctuated from 10cm below the soil surface in December to around 30–60 cm in January and in April.The mean water table started to drop off from the middle of May to 70–100 cm inJune. This conforms to the declining rainfall in these periods and suggests that, startingfrom June, the contribution of the capillary water from the groundwater to theroot zone decreased substantially. The water table depth in February-March appearedvery deep and might not adequately reflect the “medium” value because we had onlyone set of data that were measured in 1998, a year with particularly low rainfall.64 Boling et al